Magnetic data storage media includes a recording layer formed on a substrate. Data is stored on the media by changing magnetic polarities among consecutive magnetic domains in the recording layer. The domains of contemporary magnetic storage media include multiple distinct grains of a magnetic material. Denser media can be provided by forming smaller domains. However, there is a practical limit as to the extent the domains can be minimized in size and yet still be comprised of a plurality of distinct grains.
One particular effect that limits minimization of domain size is a super-paramagnetic effect. The super-paramagnetic effect occurs when the grain volume is too small to prevent thermal fluctuations from spontaneously reversing magnetization direction in the grains. One technique to delay the onset of the super-paramagnetic effect is to use bit patterned media, where each bit is a single magnetic switching volume (e.g., a single grain or a few strongly coupled grains), as described in R. D. Terris et al., J. Phys. D: Applied Physics 38, R199 (2005). In order to keep thermally activated reversal at an acceptable level, KuV/kbT, where Ku represents the magnetic anisotropy, V represents the magnetic switching volume, kb represents the Boltzmann constant, and T represents the temperature in Kelvin. The ratio must remain greater than approximately 60 for conventional longitudinal media according to D. Weller, et al. “Thermal Effect Limits in Ultrahigh-Density Magnetic Recording”, IEEE Trans. on Magnetics 35, 4923 (1999). To maintain a sufficient SNR, it is desirable to conserve the number of grains per bit as the density is increased. The switching volume in discrete dots is equal to the bit size, and dots smaller than 10 nm can be thermally stable.
A patterning process typically consists of several steps including lithography to define the pattern, and pattern transfer onto the substrate or thin film. In general, there are two classes of pattern formation processes, additive and subtractive. In the additive process (electrodeposition and lift-off), the resist pattern is first created and then the magnetic film is deposited. In the subtractive process, the magnetic film is deposited prior to resist patterning. The pattered resist then serves as an etch mask, and the surrounding magnetic film is removed by one of a number of processes including ion milling, RIE and wet chemical etching. A commonly used process for removing magnetic materials is ion milling, which is not considered to be a selective removal process. C. Ross, “Patterned Magnetic Recording Media” Annual. Rev. Mater. Res. 31, 203-35 (2001).
The modification of magnetic films through Ga+ poisoning using FIB (Focused Ion Beam) has been described in the art. With this approach, perpendicular granular media based on CoPtCr was not etched, but rather poisoned by Ga+. Islands (dots) smaller than 70 nm in diameter were seen to have a domain remnant state. However, one drawback of this method is that FIB methods lack throughput to be a low-cost manufacturing method for patterned media. C. T. Rettner, et al. “Patterning of Granular Magnetic Media with a Focused Ion Beam to Produce Single-Domain Islands at >140 Gbit/in2” IEEE Trans. on Magnetics 37, 1649 (2001); C. T. Rettner, et al. “Magnetic Characterization and Recording Properties of Patterned Co70Cr18Pt12”, IEEE Trans. on Magnetics 38, 1725 (2002); C. T. Rettner et al. Applies Physics Letters 80, 2 279 (2002); R. Hyndman et al. “Modification of Co/Pt Multilayers by Gallium Irradiation—Part 1: The Effect on Structural and Magnetic Properties” J. Appl. Phys. 90, 3843 (2001).
Another contemporary method deposits a recording layer that includes magnetic material separated by inherently non-magnetic regions, masking a surface of the recording layer where the mask covers areas desired to be used as recording domains, and then processing the exposed regions to reduce magnetism. The inherently non-magnetic regions serve to protect and preserve the isolation of the magnetic material regions after etching away the exposed magnetic material. However, this contemporary approach calls for provision of a recording layer with multiple different materials, which substantially increases the complexity of the manufacturing process.